1. Field of The Invention
The present invention relates generally to a circuit of AC/DC converter which converts a commercially available alternating current power supply voltage into a stable DC output voltage, particularly, relates to the AC/DC converter having a high power factor.
2. Description of The Background Art
FIG. 1 shows a conventional AC/DC converter which converts the received commercially available alternating current (AC) power supply into a direct current (DC) voltage.
In FIG. 1, the AC/DC converter includes an input rectifying circuit RC1 which carries out a full rectification of an input alternating current voltage E.sub.i, a smoothing capacitor C1, a field effect transistor Q1 which serves as a switching device, a transformer Tr1, transformer magnetically resetting diode D4, an output rectifying diode D2, a flywheel diode D3, an output smoothing choke coil L2, and an output smoothing capacitor C2. Thus, the AC/DC converter serves to convert the alternating input voltage E.sub.i into a stable DC output voltage E.sub.0.
Although, in such an AC/DC converter as described above, a desired characteristic is exhibited for the output DC voltage, the input current thereof I.sub.i flows only at a time at which a peak of the instantaneous input alternating current voltage E.sub.i approximately appears as appreciated from FIG. 2.
Therefore, a power factor of the AC/DC converter shown in FIG. 1 is as low as about 0.5 through 0.7.
To improve the power factor, a method of inserting a large sized choke coil into the rectifying circuit has been proposed. However, the size and weight of the AC/DC converter are too large and heavy.
Another method for improving the power factor is to install a pre-converter to electronically improve the power factor has been proposed. However, the other method introduces the increased number of parts and components, large-sized and expensive AC/DC converter, and interferences between the switching devices in the AC/DC converters.